In recent years, strip-line circuitry has found increasing applications in various items of hardware which are mass produced and which must operate under extreme environmental conditions. These applications have placed stringent requirements on the techniques used for packaging the circuitry. The strip-line packages must be capable of being assembled to produce uniform performance results not only at initial hardware acceptance but also after having been subjected to environmental extremes. Packaging techniques which have been and are being used have resulted in high rejection rates at initial hardware acceptance and degraded performance after environmental exposure.
This invention concerns the bonding of conductors to dielectric circuit boards for microwave applications and more particularly to the bonding of copper coating or foil to a substrate of glass-loaded polystyrene, divinyl benzol styrene, teflon or the like. A method of making a circuit board of the general type has been described in U.S. Pat. No. 3,318,758. However, the material used was not isotropic and had a water absorption factor which is highly objectionable in many microwave applications. Furthermore, the bonding process described in the patent necessitated the formation of a continuous coating of copper oxide on the copper sheet prior to bonding.
Numerous attempts have been made to provide a printed circuit board suitable for microwave use. Pure copper is known to be the best for use in such electrical circuits. Therefore, a laminate or printed circuit board formed of a layer of copper foil firmly bonded to a substrate of synthetic plastic material has been considered as providing an ideal board for microwave printed circuits.
The bonding of copper sheet or foil to a substrate of a dielectric material comprising a glass-loaded divinyl benzol styrene, for example, was conceived but presented many problems in the manufacture. Numerous attempts have been made by workers in this field to produce such a laminate, but to date these efforts have not been entirly successful. The relatively low softening temperature of the materials and the difficulty of bonding a proper foil to the materials without the use of cement have been given as reasons for failure. The successful manufacture of a circuit board of this configuration and the method for manufacture thereof is a subject of this invention.
A printed circuit board, in order to possess the physical and electrical characteristics required of microwave circuitry, must be stable in shape and dimension, have a low loss tangent, low isotropic dielectric constant, nil water absorption, high peel strength and chemical inertness. In addition, when the board is to be used with a wiper type switch arm, the surface of the copper should be flush with the surface of the dielectric, without a groove or trough between segments.
The method according to the present invention eliminates the use of fasteners and is accomplished without leaving air gaps and with no warping of the boards. The method produces boards having greater reliability with lower assembly costs and practically unlimited shelf life. The design performance of the boards is predictable, overall package size may be made smaller and orthogonal (shear) stresses do not exist on the circuiting.
As used herein, the term "plastic" includes any synthetic organic material of high molecular weight which, while solid in the finished state, at some stage in its manufacture is soft enough to be formed into shape by some degree of flow.
The well-known term "Teflon" as used herein is the trademark of the E. I. du Pont de Nemours and Company, Incorporated and refers to the plastic polymer tetrafluoroethylene as manufactured by them.